CN102301027A

CN102301027A - High-strength hot-dip galvanized steel sheet and manufacturing method therefor

Info

Publication number: CN102301027A
Application number: CN2010800064198A
Authority: CN
Inventors: 小野义彦; 高桥健二; 奥田金晴; 平章一郎
Original assignee: NKK Corp
Current assignee: JFE Engineering Corp
Priority date: 2009-02-02
Filing date: 2010-02-02
Publication date: 2011-12-28
Anticipated expiration: 2030-02-02
Also published as: KR101379973B1; EP2392683B1; JP2010196159A; US20120037281A1; CA2750890A1; KR101217921B1; KR101624473B1; US8636852B2; KR20130122009A; US20140102597A1; MX2011007977A; CN102301027B; KR20120105061A; JP4623233B2; KR20110105404A; EP2392683A4; WO2010087529A1; KR20130122008A; EP2392683A1; KR20150038728A

Abstract

Provided are a high-strength hot-dip galvanized steel sheet that requires no addition of large amounts of Mo, Cr or other expensive elements or any special CGL heat history, and has low YP, high BH, excellent aging resistance and excellent corrosion resistance, and a manufacturing method therefor. The steel includes, by mass%, C: more than 0.015% and less than 0.100%, Si: 0.3% or less, Mn: less than 1.90%, P: 0.15-0.05%, S: 0.03% or less, sol. Al: 0.01-0.5%, N: 0.005% or less, Cr: less than 0.30%, B: 0.0003-0.005%, and Ti: less than 0.014%, wherein 2.2 <= [Mneq] <= 3.1 and 0.42 <= 8[%P] + 150B* <= 0.73 are satisfied. The steel structure has ferrite and a second phase. The surface area ratio of the second phase is 3-15%, the ratio of martensite and residual gamma surface area ratio to the second phase surface area ratio is more than 70%, and the ratio of the surface area ratio present at the grain boundary triple points, of the second phase surface area ratio, is 50% or more.

Description

High-strength hot-dip zinc-coated steel sheet and manufacture method thereof

Technical field

The press forming high-strength hot-dip zinc-coated steel sheet and the manufacture method thereof that the present invention relates to process press forming operation in automobile, household electrical appliances etc. and use.

Background technology

In the past, the BH steel plate of TS:340MPa level (sinter-hardened shaped steel plate, below only be called 340BH) was applied to engine shield, car door, trunk lid, tail gate, fender etc. and required in the automobile exterior panel panel of shock-resistance.340BH is at C: usually control the single-phase steel of ferrite that solid solution C measures and carries out solution strengthening by Mn, P by the unit that adds formations carbonitrides such as Nb, Ti in be lower than 0.01% ultra-low carbon steel of (% is quality %, below identical).In recent years, body lightening requires further to improve, and ongoing research is: thus the further high strength of the outer plate panel that makes these use 340BH makes the steel plate thin-walled property; Perhaps, under identical thickness of slab, cut down R/F (reinforce: inboard reinforcing member) and then make sintering coating process low temperatureization, short period of timeization etc.

But, by in 340BH in the past, further add Mn in large quantities, when P realizes high strength because the increase of yielding stress (YP), the remarkable variation of anti-surface strains of press forming product.At this, surface strains is meant, easily small wrinkle, the undulatory decorative pattern of the press forming face of locating to produce in the periphery of the handle portion of car door etc.The surface strains meeting significantly damages the exterior quality of automobile, therefore, for the steel plate in being applied to outer plate panel, when requiring to improve the intensity of stampings, also requires the preceding yielding stress of press forming to have the low YP approaching with existing 340BH.

On the other hand, for the intensity improve press forming and sintering application when keeping low yielding stress after, sinter-hardened (BH) after the work hardening (WH) in the time of need making compacting, the compacting increases.Wherein, the dependent variable that is subjected to when not relying on press forming and stably guarantee high-impact preferably makes BH increase.But, when BH is increased, produce the variation of ageing resistance.Particularly in recent years because the globalization in vehicle production base,, in South East Asia, the demand of steel plate for panel such as South America, India also increasing, thereby further require the raising of ageing resistance not only in the North America or the Northeast Asian region.For example, use under the situation of steel plate under the line in the neighbourhood, consider transportation operation or the keeping period in local warehouse, because steel plate will expose 2～5 months down at 40～50 ℃, therefore, if the single-phase steel of ferrite in the past, then ageing resistance is insufficient, and the outside plate design surface after compacting produces wrinkle shape decorative pattern.Like this, in recent years,, also require to have the ageing resistance better than steel in the past as the steel plate characteristic even keep high BH.

In addition, the steel plate used of automobile also requires superior corrosion resistance.For example, in parts such as car door, engine shield, trunk lid, outer plate panel for interior bonds, its flange part is by edge processing and crooked.Perhaps, implement spot welding.Because the chemical conversion tunicle that this edge adds when carrying out driving fit and electrodeposition coating between the steel plate of the Ministry of worker or spot welding periphery is difficult to adhere to, and therefore gets rusty easily.Particularly be exposed to for a long time in the corner portion of the corner portion in the engine shield the place ahead in the moist environment and bottom of door, produce the perforation that causes by rust repeatedly accumulating water easily.Therefore, the steel plate used of outer plate panel requires superior corrosion resistance.Particularly the motor body builder makes the rustless property of vehicle body improve and is making the endurable hole life-span expand in the past the research in 10 years to 12 years in recent years, and as seen, steel plate possesses sufficient erosion resistance and is absolutely necessary.

For above-mentioned background, for example, disclosed method is in patent documentation 1, by the speed of cooling optimization after the annealing that makes the steel that contains C:0.005～0.15%, Mn:0.3～2.0%, Cr:0.023～0.8% and form the complex tissue that mainly constitutes, obtain having concurrently the method for the alloy galvanized steel plate of low yielding stress (YP), high sinter-hardened (BH) by ferrite and martensite.

In addition, disclosed method is in the patent documentation 2, by containing C: be higher than 0.01% and be lower than 0.03%, add 0.02～1.5% Mo and control sol.Al, N in Mn:0.5～2.5%, the steel below the B:0.0025%, B, Mn amount makes sol.Al 〉=9.7 * N, B 〉=1.5 * 10 ⁴* (Mn ²+ 1) thus obtain generating the tissue that constitutes mutually by ferrite and low temperature phase change, obtain the method for sinter-hardened property and the two all good hot-dip galvanizing sheet steel of normal temperature ageing resistance thus.

Disclosed method is in the patent documentation 3, by to contain more than the C:0.005% and be lower than 0.04%, the steel plate of Mn:0.5～3.0% carries out in the hot rolled process being cooled to below 650 ℃ with the speed of cooling more than 70 ℃/second with interior back 2 seconds of rolling end, obtains the good steel plate of ageing resistance thus.

Disclosed method is in the patent documentation 4, by contain C:0.02～0.08%, Mn:1.0～2.5%, below the P:0.05%, Cr: be higher than that to make Cr/Al in the steel below 0.2% and 1.5% be more than 30, obtain having low yielding ratio thus, the method for the steel plate of high BH, good normal temperature ageing resistance.

Disclosed method is in the patent documentation 5, when in the steel that contains C:0.005～0.04%, Mn:1.0～2.0%, Cr:0.2～1.0%, Mn+1.29Cr being controlled at 2.1～2.8, add Cr morely, obtain the method for the low and hot-dip galvanizing sheet steel that BH is high of YP thus.

Disclosed method is in the patent documentation 6, by will contain more than the C:0.01% and be lower than 0.040%, Mn:0.3～1.6%, steel below the Cr:0.5%, below the Mo:0.5% anneals then and to be cooled to 550～750 ℃ temperature and to be cooled to temperature below 200 ℃ with the speed of cooling more than 100 ℃/second with 3～20 ℃/second speed of cooling, obtains the method for the good steel plate of sinter-hardened property thus.

Patent documentation 1: Japanese Patent Publication 62-40405 communique

Patent documentation 2: TOHKEMY 2005-8904 communique

Patent documentation 3: TOHKEMY 2005-29867 communique

Patent documentation 4: TOHKEMY 2008-19502 communique

Patent documentation 5: TOHKEMY 2007-211338 communique

Patent documentation 6: TOHKEMY 2006-233294 communique

Summary of the invention

But, above-mentioned patent documentation 1～5 described steel plate, all based on the complex tissue steel of ferrite and martensite tissue as steel plate, steel with such tissue, if heavy addition has as the Mo of high valence elements or the steel of Cr, then compare and have fully low YP and abundant high BH, still with the steel plate of in the past solution strengthening type, if the steel that the addition of Mo, Cr is few then is difficult to obtain to have concurrently the steel of fully low YP and fully high BH.For example, in the existing steel, if be added with the steel of the Cr more than the Mo or 0.30% 0.2% or more, then the steel plate of TS:440MPa level can obtain about 250MPa or its following low YP and about 50MPa or its above high BH, still, if the few steel plate of Mo or Cr, then or the YP height, perhaps BH is low.

In addition, the described existing steel of above-mentioned patent documentation, ageing resistance also may not be abundant.For example, suppose near under the line area use steel plate, keep postevaluation in 3 months to have or not the yield-point after manifesting timeliness to increase (YPE1) down at 50 ℃ patent documentation 3 described steel plates, still not necessarily demonstrate good result.This can think because, patent documentation 3 described aging conditions be 100 ℃ 10～15 hours down, this aging condition is if be scaled 50 ℃, then at the most 0.8～1.2 month, therefore above-mentioned aging condition was also insufficient.In addition, patent documentation 3 described methods need special chilling after hot rolling, therefore, also are difficult in the common rolling line with special fast cooling device and use.In addition, put down in writing as patent documentation 2, in order to improve ageing resistance, the technology of adding a large amount of Mo of about 0.2% is a lot of in the prior art, and the manufacturing cost of such steel is significantly high.

In addition, similarly for above-mentioned patent documentation 1～6 described steel plate, the edge of having investigated simulated engine cover or car door adds the erosion resistance of the steel plate shape of the Ministry of worker, and the result as can be known, wherein the erosion resistance of most steel may not be abundant, and several steel are wherein compared the erosion resistance significance difference with existing steel.

In addition, about patent documentation 6 described methods, owing to need after annealing, carry out chilling, therefore can apply in the continuous annealing line of handling (CAL) in unreal plating and use, but, be difficult in the existing continuous hot galvanizing line (CGL) and use, described existing continuous hot galvanizing line is impregnated in the cooling after annealing and implements the plating processing in the zinc-plated bath that remains on 450～500 ℃.

The present invention finishes in order to solve such problem, its purpose is, high price elements such as not needing heavy addition Mo or Cr is provided or does not need special CGL thermal history and has low YP, high BH, good ageing resistance, the high-strength hot-dip zinc-coated steel sheet and the manufacture method thereof of superior corrosion resistance.

The inventor is an object with the low complex tissue steel plate of existing yield strength, for when improving erosion resistance, not using high valence elements and guaranteeing low YP, high BH simultaneously and the method for good ageing resistance has been carried out research extensively and profoundly, obtained following conclusion.

(I) in order to keep the low intensive hardening capacity of guaranteeing simultaneously, the relatively large Cr that is added with in the existing complex tissue steel plate, still, the edge adds the erosion resistance of the Ministry of worker because the interpolation of Cr and remarkable variation.Therefore, the erosion resistance in order to ensure more than equal with 340BH need make Cr content be reduced to and be lower than 0.30%.

(II) in order to suppress YP or yield ratio (YR) lower and to guarantee good ageing resistance, need raising Mn equivalent and suppress pearlitic generation, thereby be controlled to be by ferrite be mainly martensitic second complex tissue that constitutes mutually, and guarantee that simultaneously the area occupation ratio of second phase is more than 3%.

(III) from guaranteeing the viewpoint of erosion resistance,, for example need to effectively utilize Mn in order when reducing Cr, to guarantee sufficient Mn equivalent, but, during heavy addition Mn, become inhomogeneous thereby ferrite crystal grain stretches size-grade distribution, and the remarkable microminiaturization of martensite causes the increase of YP.Relative therewith, the effect that B (boron) or P (phosphorus) improve hardening capacity is remarkable, and have make that ferrite crystal grain is even, the effect of thick ground polygonization with make second effect that is evenly dispersed in the triple point place of ferrite crystal boundary mutually.Particularly, B has very strong making, and ferrite crystal grain is even, the effect of thickization, and P has the very strong homodisperse effect of martensite that makes.Therefore, by compound interpolation P and B within the limits prescribed, and the addition of Mn is suppressed in the specialized range, can obtains simultaneously thus evenly, coarse ferrite grains and homodisperse martensite crystal grain, even and the component steel that has reduced Cr or Mo also can obtain low YP.

(IV) heavy addition of Mn is owing to the minimizing of solid solution C and second mutually the inhomogeneous decentralized remarkable variation of BH that makes.On the other hand, logical P and the B itself of adding has the effect that increases BH.Therefore, by adding P and B and cut down the addition of Mn more than specified amount, BH significantly increases.Therefore, be controlled in the specified range, can obtain low YP and high BH simultaneously by control Mn equivalent and with P, B, Mn.

(V) effectively utilize P and B and improve the normal steel of the present invention of Mn, ferrite transformation postpones in the process of cooling after hot rolling, therefore need not to implement special chilling and chilling by implementing appropriateness and in the temperature range of regulation, implement to batch processing, hot rolling microstructure becomes small ferrite and small perlite or bainite, cold rolling, annealing after homogeneous microstructureization, BH further improves.

Like this, by being reduced to, Cr is lower than 0.30%, improve the Mn equivalent simultaneously, add P and B with specified amount compoundly and the addition of Mn is controlled in the specialized range, and then, can access the steel that has superior corrosion resistance, low YP, high BH and good ageing resistance concurrently with the speed of cooling optimization after the hot rolling.And, owing to do not use Mo or the such high valence elements of Cr, therefore, can make at an easy rate, and not need special thermal history.

The present invention is based on above opinion and finish, a kind of high-strength hot-dip zinc-coated steel sheet is provided, it is characterized in that, one-tenth as steel is grouped into, in quality %, contain C: surpass 0.015% and be lower than 0.100%, below the Si:0.3%, Mn: be lower than 1.90%, more than the P:0.015% and below 0.05%, below the S:0.03%, more than the sol.Al:0.01% and below 0.5%, below the N:0.005%, Cr: be lower than 0.30%, above and the 0.005% following and Ti of B:0.0003%: be lower than 0.014%, and satisfy 2.2≤[Mneq]≤3.1 and 0.42≤8[%P]+150B ^*≤ 0.73, surplus is made of iron and unavoidable impurities; Tissue as steel, have ferrite with second mutually, the area occupation ratio of second phase is 3～15%, and the area occupation ratio of martensite and remaining γ is more than 50% with respect to the ratio that the ratio of the second phase area rate surpasses the area occupation ratio of second phase that is present in crystal boundary triple point place in 70%, the second phase area rate.

At this, [Mneq]=[%Mn]+1.3[%Cr]+8[%P]+150B ^*, B ^*=[%B]+[%Ti]/48 * 10.8 * 0.9+[%Al]/27 * 10.8 * 0.025, the content separately of [%Mn], [%Cr], [%P], [%B], [%Ti], [%Al] expression Mn, Cr, P, B, Ti, sol.Al, B ^*〉=0.0022 o'clock, B ^*=0.0022.

High-strength hot-dip zinc-coated steel sheet of the present invention is below the preferred Mo:0.1%.

High-strength hot-dip zinc-coated steel sheet of the present invention preferably satisfies 0.48≤8[%P]+150B ^*≤ 0.73.

In addition, in quality %, contain preferably that V:0.4% is following, Nb:0.015% following, W:0.15% is following, Zr:0.1% is following, Cu:0.5% is following, Ni:0.5% is following, Sn:0.2% is following, Sb:0.2% is following, Ca:0.01% is following, Ce:0.01% is following, La:0.01% at least a kind in following.

High-strength hot-dip zinc-coated steel sheet of the present invention, manufacture method manufacturing by high-strength hot-dip zinc-coated steel sheet, the method is characterized in that, the steel billet that will have the mentioned component composition carries out hot rolling and cold rolling, then in continuous hot galvanizing line (being CGL), be higher than 740 ℃ and be lower than under 840 ℃ the annealing temperature and anneal, cooling off from described annealing temperature average cooling rate with 2～30 ℃/second before be impregnated into the zinc-plated bath, be impregnated into then carry out in the zinc-plated bath zinc-plated, zinc-plated back is cooled to below 100 ℃ with 5～100 ℃/second average cooling rate, perhaps, zinc-plated back is further implemented the Alloying Treatment of coating and is cooled to below 100 ℃ at the average cooling rate with 5～100 ℃/second after the Alloying Treatment.

The manufacture method of high-strength hot-dip zinc-coated steel sheet of the present invention is cooled to batch under 400～620 ℃ then below 640 ℃ with the average cooling rate more than 20 ℃/second after the preferred hot rolling.

The invention effect

According to the present invention, can be under the situation that need not special CGL thermal history make excellent corrosion resistance, YP is low, BH is high and ageing resistance is also good high-strength hot-dip zinc-coated steel sheet with low cost.High-strength hot-dip zinc-coated steel sheet of the present invention owing to have superior corrosion resistance, good anti-surface strains, good shock-resistance, good ageing resistance concurrently, therefore can be realized high strength, the thin-walled property of trolley part.

Description of drawings

Fig. 1 is expression YP and 8P+150B ^*The figure (P represents [%P]) of relation.

Fig. 2 is expression BH and 8P+150B ^*The figure (P represents [%P]) of relation.

Fig. 3 is the figure of the relation of expression YP and P amount.

Fig. 4 is the figure of the relation of expression BH and P amount.

Fig. 5 is expression YP, BH and Mn, 8P+150B ^*The figure (P represents [%P]) of relation.

Fig. 6 is the figure of the relation of the average cooling rate till 640 ℃ and BH after the expression hot rolling.

Embodiment

Below explain the present invention.In addition, the % of the amount of expression composition does not then represent quality % as long as there is special instruction.

1) one-tenth of steel is grouped into

Cr: be lower than 0.30%

Cr is the important element that needs strict control in the present invention.That is, in the past, for reducing YP, improve the purpose of BH and actively effectively utilize Cr, but Cr was not only the element of high price, and made the edge add the remarkable variation of erosion resistance of the Ministry of worker when clearly learning its heavy addition.That is, door skin, the engine hood outer panel parts done with the low complex tissue steel of existing YP are estimated in the erosion resistance under the moist environment, be found that the steel plate that its perforation life-span that adds the Ministry of worker for the edge reduces 1～4 year than existing steel.And clear and definite such erosion resistance variation is 0.30% to take place when above at the content of Cr, and significantly takes place when above 0.40%.Therefore, in order to ensure sufficient erosion resistance, need make the content of Cr be lower than 0.30%.From the viewpoint that shown below making [Mneq] optimized, the element of Cr for adding arbitrarily, and do not have lower limit regulation (comprising Cr:0%), but from hanging down the viewpoint of YPization, Cr preferably adds more than 0.02%, more preferably adds more than 0.05%.

[Mneq]: more than 2.2 and below 3.1

In order when guaranteeing high BH, to guarantee low YP and good ageing resistance, need make structure of steel be by ferrite be mainly the martensitic complex tissue that constitutes mutually.Common YP or YR fully do not reduce in the existing steel steel plate or the inadequate steel plate of ageing resistance are investigated its reason, and the result shows that in such steel plate, as second phase, except that martensite and a spot of remaining γ, also generating has perlite, bainite.This perlite is small, is about 1 μ m～about 2 μ m, and generates in abutting connection with martensite, therefore is difficult under opticmicroscope and the martensitic phase difference, can discern by using SEM to observe with the multiplying power more than 3000 times.For example, when the tissue of existing 0.03%C-1.5%Mn-0.5%Cr steel is carried out The effect, by the observation under opticmicroscope or with the observation of the use SEM of about 1000 times multiplying power, only can discern thick perlite, thereby the area occupation ratio that can measure perlite or bainite is about 10% with respect to the area occupation ratio of second phase, but when carrying out The effect by 4000 times SEM observation, perlite or the bainite ratio shared with respect to the area occupation ratio of second phase is 30～40%.By suppressing such perlite or bainite, can when guaranteeing high BH, obtain low YP.

In order to make so small perlite or bainite, the hardening capacity of various elements is investigated implementing fully reduction in the slow refrigerative CGL thermal history after the annealing.Found that except that the Mn, the Cr that up to the present extensively know as the hardening capacity element, B, P is the be greatly improved effect of hardening capacity of tool also.In addition we know, during the compound interpolation of B and Ti or Al, the effect that improves hardening capacity significantly increases, but the raising effect of hardening capacity is saturated when adding more than specified amount, so their effect is as shown in the formula representing with the form of Mn equivalent formula.

[Mneq]＝[％Mn]+1.3[％Cr]+8[％P]+150B ^*

B ^*＝[％B]+[％Ti]/48×10.8×0.9+[％Al]/27×10.8×0.025

Wherein, [%B]=0 o'clock, B ^*=0; B ^*〉=0.0022 o'clock, B ^*=0.0022.

At this, the content separately of [%Mn], [%Cr], [%P], [%B], [%Ti], [%Al] expression Mn, Cr, P, B, Ti, sol.Al.

B ^*Be that expression makes the residual index that improves the effect of hardening capacity of solid solution B by interpolation B, Ti, Al, do not add the steel of B, can not obtain adding the effect that B brings, so B ^*=0.In addition, B ^*Be 0.0022 when above, the raising effect of the hardening capacity that B brings is saturated, so B ^*Be 0.0022.

By making this [Mneq] is more than 2.2, implements in the slow refrigerative CGL thermal history after annealing, and perlite or bainite are also fully suppressed.Therefore, in order to obtain good ageing resistance when reducing YP, need make [Mneq] is more than 2.2.In addition, from the viewpoint of low YPization, [Mneq] is preferably more than 2.3, more preferably more than 2.4.[Mneq] surpasses at 3.1 o'clock, and the addition of Mn, Cr, P is too much, is difficult to guarantee simultaneously low fully YP, high BH, superior corrosion resistance.Therefore, [Mneq] is made as below 3.1.

Mn: be lower than 1.90%

As mentioned above, carry out high BHization when hanging down YPization, need to carry out the optimization of [Mneq] at least, but only this is inadequate, also need the content of Mn amount and P described later, B is controlled in the specialized range.That is, the purpose of adding Mn is, improves hardening capacity and increases by second the martensitic ratio in mutually.But, when its content is too much, α in the annealing process → γ transformation temperature reduces, the small ferrite crystal boundary behind firm recrystallize or the γ of the generation at the interface crystal grain of the regenerated particle in the recrystallization process, therefore, the ferrite crystal grain exhibition is stretched and is become inhomogeneous, thus and second phase microminiaturization YP rising.Simultaneously, the interpolation of Mn, owing to make the Al alignment low temperature of Fe-C state graph, low C side shifting, thereby have the solid solution C minimizing that makes in the ferrite and make the second dispersive effect unevenly mutually, therefore BH is significantly reduced.

Therefore, in order to obtain low YP and high BH simultaneously, need make the Mn amount be lower than 1.90%.In order further to realize low YPization and high BHization, preferably making the Mn amount is below 1.8%.In addition, in order to bring into play the effect of such Mn, the preferred interpolation surpasses 1.0% Mn.

More than the P:0.015% and below 0.05%

P is the important element that is used to realize low YPization and high BHization in the present invention.That is, P can realize low YPization, high BHization and good ageing resistance simultaneously with low manufacturing cost, and can guarantee superior corrosion resistance by being used in combination and containing in specialized range with B described later.

P was used effectively as the solution strengthening element in the past, and from the viewpoint of low YPization, still expectation reduces its content.But, as mentioned above,, also have the effect of significant raising hardening capacity even known P trace adds.In addition, known P has the effect that makes the second mutually even and thick dispersive effect at the triple point place of ferrite crystal boundary and BH is increased a little.Therefore, the hardening capacity of effectively utilizing P is improved effect and the method for low YPization, high BHization that realizes has been carried out research extensively and profoundly.Found that,, can make second to disperse extremely equably mutually, thereby BH significantly improves when YP reduces by in [Mneq] that keep regulation, replacing Mn with P.

And P is the element that erosion resistance is improved a little, therefore by with P replaced C r, can in the good material of maintenance erosion resistance be improved.The effect of bringing in order to obtain such interpolation P, P needs to add more than 0.015% at least, preferably adds more than 0.02%.

But, when interpolation surpasses 0.05% P, improves the effect of hardening capacity and homogenizing, the thickization effect of tissue and reach capacity, and the solution strengthening quantitative change is too much, thereby can not obtain low YP.In addition, the increase effect of BH also diminishes.In addition, when interpolation surpasses 0.05% P, thereby the alloying reaction of base steel and coating significantly postpones the chalking resistance variation.In addition, also variation of weldability.Therefore, the P amount is made as below 0.05%.

More than the B:0.0003% and below 0.005%

B has makes that ferrite crystal grain is even, the effect of thickization, make effect that hardening capacity improves and the effect that BH is increased.Therefore, by when guaranteeing [Mneq] of specified amount, replacing Mn, can realize low YPization and high BHization with B.Make the P and B with effect of making ferrite crystal grain even thickization of martensite by being used in combination to have in the effect of crystal boundary place generation, can access by even coarse ferrite grains and the structure of steel that constitutes at its homodisperse martensite in crystal boundary triple point place, and can significantly realize the reduction of YP, the raising of BH.In order to obtain the effect that such B adds, it is more than 0.0003% that B needs at least.In order further to bring into play the effect of adding the low YPization that B brought, B preferably adds more than 0.0005%, further preferred the interpolation above 0.0010%.But, to add when surpassing 0.005% B, castibility, rolling property significantly reduce.Therefore, B is made as below 0.005%.From guaranteeing the viewpoint of castibility, rolling property, preferably add the B below 0.004%.

0.42≤8[％P]+150B ^*≤0.73

In order to realize except that the content separately of P, B, Mn, also needing low YPization and high BHization simultaneously with P and B ^*The weighting equivalent formula be controlled in the specialized range and be optimized.Therefore, at first, investigated the variation that makes [Mneq] certain and mechanical characteristics when adding P and B.Chemical ingredients for the examination steel is C:0.027%, Si:0.01%, Mn:1.5～2.2%, P:0.004～0.05%, S:0.003%, sol.Al:0.05%, Cr:0.20%, N:0.003%, B:0.0005～0.0018%, and make the addition of the addition of Mn and P, B keep balance in [Mneq] roughly certain mode in 2.5 to 2.6 scope, the steel that obtains is like this carried out vacuum melting.In addition, as a comparison, following component steel is melted together: P:0.01%, B: do not add, the Mn of Mn:2.2%, Cr:0.20% is the component steel of main body; P:0.01%, B: do not add, the interpolation of Mn:1.6%, Cr:0.65% the component steel of Cr; With P:0.01%, B:0.001%, Mn:1.6%, Cr: do not add, the interpolation of Mo:0.2% the component steel of Mo.In addition, Mn is that the component steel and the Cr of main body is the component steel of main body, and is same with P, B interpolation steel, and [Mneq] is adjusted to 2.5～2.6.

Cut out the thick steel billet of 27mm from resulting steel ingot, be heated to after 1200 ℃ and under 850 ℃ finishing temperature, be hot-rolled down to 2.8mm, the cooling of spraying water immediately after rolling, and 570 ℃ of processing of batching of implementing 1 hour down.The hot-rolled sheet that obtains is cold-rolled to 0.75mm with 73% rolling rate.To 780 ℃ * 40 seconds the annealing of cold-reduced sheet enforcement that obtains, begin to cool off from annealing temperature with 7 ℃/second average cooling rates, be impregnated into and implement pot galvanize in 460 ℃ the zinc-plated bath and handle, then, for being carried out Alloying Treatment, coating kept 15 seconds down at 510 ℃, be cooled to temperature range below 100 ℃ with 25 ℃/second speed of cooling then, and implement temper rolling with 0.2% elongation.

Cut No. 5 tension test sheets of JIS from the steel plate that obtains, implement tension test (according to JISZ2241).In addition, measure the stress give after 2% the prestrain, with give 2% prestrain and and then poor at 170 ℃ of upper yield stresses after implementing the thermal treatment suitable in 20 minutes down with the sintering coating process, with it as BH.

The results are shown in Fig. 1 and Fig. 2 with what obtain.At this, ◆ be illustrated in the mechanical characteristics of the steel that adds P in the fewer component steel of the B addition of B:0.0005～0.0010% and obtain, ◇ is illustrated in the mechanical characteristics of the steel that adds P in the many component steel of the B addition of B:0.0013～0.0018% and obtain.In addition, * expression Mn be the mechanical characteristics of the component steel of main body, mechanical characteristics that zero expression Cr is the component steel of main body, ● the mechanical characteristics of steel of Mo has been added in expression.Thus, 8[%P]+150B ^*Be more than 0.42, YP reduces, and BH significantly increases simultaneously.In addition, 8[%P]+150B ^*Be 0.48 when above, when keeping low YP, can obtain higher BH.It is the steel of main body and low and approaching with the steel that the has added Cr low value of steel that has added Mo that the YP of this moment demonstrates than Mn.In addition, Ci Shi BH demonstrates than Mn and is that the steel of main body significantly improves and adds steel with Cr and Mo adds the equal or above value of steel.In addition, Fig. 3, Fig. 4 show the many component steel (B of B addition of above-mentioned B:0.0013～0.0018% ^*Roughly certain steel in 0.0019～0.0022) and be used for comparison and the Mn that illustrates be the component steel of main body, component steel that Cr is main body, the relation component steel of having added Mo, YP and P amount, BH and P amount.The making method of sample is identical with the method for Fig. 1, Fig. 2.Hence one can see that, and interpolation P cuts down Mn in the steel by adding at B, can keep low YP and obtain high BH.In addition we know, in order to obtain such effect, it is more than 0.015% that P needs at least.In addition, above-mentioned steel all has the intensity of TS 〉=440MPa.

Therefore, for further clear and definite suitable Mn amount and 8[%P]+150B ^*Scope, investigated the mechanical characteristics of the steel that the composition that makes Mn and P, B significantly changes.And the chemical ingredients beyond Mn, P, the B and the making method of sample are same as described above.With the Fig. 5 that the results are shown in that obtains.Among the figure, the steel plate of YP＜215MPa and BH 〉=60MPa with ● expression, is represented the steel plate of 215MPa≤YP≤220MPa and BH 〉=60MPa with △, with the steel plate of YP≤220MPa and 55MPa≤BH＜60MPa with zero expression.In addition, with the steel plate that do not satisfy the YP＞220MPa of above-mentioned characteristic or BH＜55MPa with ◆ expression.

Hence one can see that, satisfy that [Mneq] is 2.2 or more, Mn measure be lower than 1.90% and 0.42≤8[%P]+150B ^*, can obtain low YP and high BH simultaneously at≤0.73 o'clock.In addition, satisfy 0.48≤8[%P]+150B ^*The time, can access higher BH.In addition, be more than 2.3 and 8[%P by making [Mneq]]+150B ^*Be below 0.70, can access lower YP and the BH of Geng Gao.Such steel plate has with ferrite for leading and comprise martensitic tissue, and the growing amount of perlite and bainite reduces.In addition, ferrite crystal grain is even, thick, and martensite is mainly at the triple point place of ferrite crystal grain homodisperse.But, 8[%P]+150B ^*Surpass at 0.73 o'clock, the addition of P need surpass 0.05%, and therefore, though homogeneous microstructureization, the solution strengthening of P becomes excessive, thereby can not obtain fully low YP.

Since above situation, 8[%P]+150B ^*Be more than 0.42 and below 0.73, preferred more than 0.48 and below 0.73, further preferred more than 0.48 and below 0.70.

C: surpass 0.015% and be lower than 0.100%

C is the bioelement of area occupation ratio that is used to guarantee second phase of specified amount.C measures when very few, can not guarantee the area occupation ratio of sufficient second phase, can not obtain sufficient ageing resistance and low YP.In order to obtain and the equal above ageing resistance of existing steel, need make C surpass 0.015%.From further raising ageing resistance and further reduce the viewpoint of YP, preferably making C is more than 0.02%.On the other hand, C amount is 0.100% when above, and the area occupation ratio of second phase is too much, and YP increase and BH also reduce.In addition, also variation of weldability.Therefore, C amount is made as is lower than 0.100%.In order when obtaining lower YP, to obtain high BH, preferably make the C amount be lower than 0.060%, further preferably be lower than 0.040%.

Below the Si:0.3%

Si by trace add have postpone in hot rolling that scale generates the effect improving the effect of surface quality, make the alloying reaction moderate retardation of base steel and zinc in plating bath or in the Alloying Treatment, makes the microtexture of steel plate more even, the effect of thickization etc., therefore, can add from such viewpoint.But the addition of Si surpasses at 0.3% o'clock, and coating exterior quality variation is difficult to be applied to outer plate panel, and causes the rising of YP, therefore, the Si amount is made as below 0.3%.From further raising surface quality and reduce the viewpoint of YP, preferably make Si be lower than 0.2%.The element of Si for adding arbitrarily, its lower limit is regulation (comprising Si:0%) not, but from above-mentioned viewpoint, Si preferably adds more than 0.01%, further preferably adds more than 0.02%.

Below the S:0.03%

Therefore S can contain containing the effect that the separability that has a scale that makes steel plate sometimes improves, makes the raising of coating exterior quality in right amount.But, the content of S for a long time, the MnS that separates out in the steel is too much, the ductility such as stretchiness, stretch flange of steel plate is reduced, and press formability is reduced.In addition, when hot rolling steel billet, make the hot rolling reduction easily, and be easy to generate surface imperfection.In addition, erosion resistance is reduced a little.Therefore, the S amount is made as below 0.03%.From the viewpoint that ductility and erosion resistance are improved, preferably making S is below 0.02%, more preferably below 0.01%, more preferably below 0.002%.

More than the sol.Al:0.01% and below 0.5%

Al is that the hardening capacity that promotes B for fix N improves effect, ageing resistance is improved, reduces inclusion and surface quality is improved and add.The effect of the raising hardening capacity of Al is medium and small at the steel that does not add B, is about 0.1 times～about 0.2 times of Mn, still, in the steel that has added B, owing to have thereby N is fixed the effect that makes solid solution B residual with the form of AlN, even the addition of a spot of sol.Al, the effect that improves hardening capacity is also big.Otherwise, if the content of sol.Al is not optimized, then can not obtain the effect of the raising hardening capacity of B, solid solution N is residual, and ageing resistance is variation also.Improve the viewpoint that effect and ageing resistance improve from the hardening capacity that makes B, the content of sol.Al is made as more than 0.01%.In order further to bring into play such effect, the content that preferably makes sol.Al is more than 0.015%, and is further preferred more than 0.04%.On the other hand, the addition of sol.Al surpasses at 0.5% o'clock, and the effect that makes the residual effect of solid solution B and ageing resistance is improved is saturated, vainly causes cost to increase.In addition, make the castibility variation, thereby make the surface quality variation.Therefore, sol.Al is made as below 0.5%.From guaranteeing the viewpoint of good surface quality, preferably make sol.Al be lower than 0.2%.

Below the N:0.005%

N is the element that forms nitride such as BN, AlN, TiN in steel, owing to form BN, has the drawback of the effect disappearance that makes B.In addition, form small AlN and make the grain growing reduction, cause the rising of YP.In addition, when solid solution N is residual, the ageing resistance variation.From such viewpoint, must carry out strictness control to N.N content surpasses at 0.005% o'clock, and the hardening capacity that can not fully obtain B improves effect, and YP rises.In addition, such component steel, the ageing resistance variation, the applicability in outer plate panel is insufficient.Therefore, the content with N is made as below 0.005%.Thereby further reduce the viewpoint of YP from the amount of separating out that effectively utilizes B and reduce AlN, preferably making N is below 0.004%.

Below the Mo:0.1%

The viewpoint that suppresses pearlitic generation from hardening capacity is improved, hangs down YRization or in the good ageing resistance of maintenance BH is improved can be added Mo.But Mo is the element of high valency, and therefore, its addition causes significant cost to increase for a long time.In addition, when the addition of Mo increased, YP increased.Therefore, add under the situation of Mo,, the addition of Mo is defined as below 0.1% (comprises Mo:0%) from the reduction of YP and the viewpoint of cost degradation.Viewpoint from further low YPization is preferably below 0.05%, does not further preferably add Mo (below 0.02%).

Ti: be lower than 0.014%

Ti have fix N and improve the hardening capacity of B effect, make effect that ageing resistance improves and the effect that castibility is improved, in order to obtain such effect complementaryly, Ti is the element that can choose interpolation wantonly.But, it contains quantitative change for a long time, have in steel, form TiC, Ti (C, N) thereby thereby etc. small precipitate YP is significantly risen and the cooling after annealing in generate the effect that TiC reduces BH, therefore, under the situation of adding, the content of Ti need be controlled in the proper range.The content of Ti is 0.014% when above, and YP significantly increases and BH significantly reduces.Therefore, the content of Ti is made as was lower than for 0.014% (comprising Ti:0%).For the fix N by separating out of TiN, thereby the hardening capacity of performance B improves effect, and the content that preferably makes Ti is more than 0.002%, thereby obtains low YP and high BH in order to suppress separating out of TiC, preferably makes the content of Ti be lower than 0.010%.

Surplus is iron and unavoidable impurities, but also can contain the following element of specified amount.

Below the V:0.4%

V is the element that hardening capacity is improved, and makes the effect of coating quality and erosion resistance variation little, therefore, can replace Mn and Cr effectively to utilize.From above-mentioned viewpoint, preferably add the V more than 0.005%, further preferably add the V more than 0.03%.But addition surpasses at 0.4% o'clock, causes significant cost to increase, and therefore, preferably adds the V below 0.4%.

Below the Nb:0.015%

Nb has to make and organizes grain refined and NbC, Nb (C, N) are separated out and strengthen the effect of steel plate and the effect that BH is increased by grain refined, therefore, from the viewpoint of high strength, high BHization, can add.From above-mentioned viewpoint, preferably add the Nb more than 0.003%, further preferably add the Nb more than 0.005%.But addition surpasses at 0.015% o'clock, and YP significantly rises, and therefore, preferably adds the Nb below 0.015%.

Below the W:0.15%

W can be used as hardening capacity element, precipitation strength element and effectively utilizes.From above-mentioned viewpoint, W preferably adds more than 0.01%, further preferably adds more than 0.03%.But, when its addition is too much, cause the rising of YP, so W adds below 0.15% preferably.

Below the Zr:0.1%

Zr equally also can be used as the hardening capacity element, the precipitation strength element effectively utilizes.From above-mentioned viewpoint, Zr preferably adds more than 0.01%,, further preferably add more than 0.03%.But, when its addition is too much, cause the rising of YP, so Zr adds below 0.1% preferably.

Below the Cu:0.5%

Cu improves erosion resistance a little, therefore adds from the viewpoint that improves erosion resistance is preferred.In addition, Cu is the element of sneaking into when steel scrap is effectively utilized as raw material, by allowing sneaking into of Cu, twice-laid stuff effectively can be utilized as raw material, thereby can cut down manufacturing cost.From above-mentioned viewpoint, Cu preferably adds more than 0.02%, and in addition, from the viewpoint of further raising erosion resistance, Cu preferably adds more than 0.03%.But, when its content is too much, become the reason of surface imperfection, therefore preferably making Cu is below 0.5%.

Below the Ni:0.5%

Ni also is the element with the effect that improves erosion resistance.In addition, Ni is containing under the situation of Cu, has the effect that reduces the surface imperfection that is easy to generate.Therefore, from above-mentioned viewpoint, Ni preferably adds more than 0.01%, improves the viewpoint of surface quality when erosion resistance is improved, and Ni further preferably adds more than 0.02%.But when the addition of Ni was too much, the scale in the process furnace generated the reason that becomes inhomogeneous and become surface imperfection, and enlarges markedly cost.Therefore, Ni is made as below 0.5%.

Below the Sn:0.2%

From to nitrogenize, oxidation or the steel plate top layer of surface of steel plate owing to the decarburization in the tens of microns zones that oxidation produces, take off the viewpoint that B suppresses, preferably add Sn.Thus, can improve fatigue characteristic, ageing resistance, surface quality etc.From suppressing nitrogenize, oxidation viewpoint, Sn preferably adds more than 0.005%, surpasses at 0.2% o'clock, causes rising, the flexible variation of YP, and therefore, Sn preferably contains below 0.2%.

Below the Sb:0.2%

Sb is also same with Sn, to nitrogenize, oxidation or the steel plate top layer of surface of steel plate owing to the decarburization in the tens of microns zones that oxidation produces, take off the viewpoint that B suppresses, the preferred interpolation.By suppressing such nitrogenize or oxidation, can prevent that martensitic growing amount reduces in the steel plate top layer.The reduction of the hardening capacity that causes by the minimizing that prevents B can improve fatigue characteristic and ageing resistance.In addition, can improve the wettability of hot galvanized layer, thereby the coating exterior quality is improved.From suppressing the viewpoint of nitrogenize or oxidation, Sb preferably adds more than 0.005%.Surpass at 0.2% o'clock, cause rising, the flexible variation of YP, so Sb contains below 0.2% preferably.

Below the Ca:0.01%

Ca has the S in the steel is fixed with the CaS form, and the pH in the corrodibility biology is increased, thereby makes the edge add the effect that the erosion resistance around the Ministry of worker and the spot welding part improves.In addition, by the generation of CaS, inhibition makes the generation of the MnS of stretch flange reduction, thereby has the effect that makes the stretch flange raising.From such viewpoint, Ca preferably adds more than 0.0005%.But Ca easy form come-up with oxide compound in molten steel is separated, thereby is difficult in steel residual in a large number.Therefore, the content with Ca is made as below 0.01%.

Below the Ce:0.01%

Ce can be that purpose is added with the S in the fixing steel also.But, owing to be the element of high price, so during heavy addition, cost increases.Therefore, from above-mentioned viewpoint, Ce preferably adds more than 0.0005%, and Ce preferably adds below 0.01%.

Below the La:0.01%

La can be that purpose is added with the S in the fixing steel also.From above-mentioned viewpoint, La preferably adds more than 0.0005%.But, owing to be the element of high price, so during heavy addition, cost increases.Therefore, La preferably adds below 0.01%.

2) tissue

Steel plate tissue of the present invention mainly is made of remaining γ, perlite, the bainite of ferrite, martensite, trace, comprises the carbide of trace in addition.At first the measuring method to these tissue morphologies describes.

The area occupation ratio of second phase is following to be obtained: after the L cross section (vertical cross-section parallel with rolling direction) of steel plate is ground, use nital to corrode, and use SEM to observe 10 visuals field with 4000 times multiplying power, the photo of taking of organizing is carried out image analysis, thereby obtain the area occupation ratio of second phase.Organize in the photo, ferrite is the zone of black slightly contrast, and the zone that carbide is generated with wire or point range shape is as perlite and bainite, and the particle that will have white contrast is as martensite or remaining γ.In addition, the following small point-like particle of the diameter 0.4 μ m that sees on the SEM photo is mainly carbide by tem observation, in addition, their area occupation ratio is very little, therefore, think to almost not influence of material, at this, do not carry out the evaluation of area occupation ratio or median size for the particle below the particle diameter 0.4 μ m, and obtain area occupation ratio to contain mainly as the particle of the white contrast of the martensitic remaining γ that contains trace with as the object that is organized as of the carbide of the line of perlite and bainite or point range shape.The area occupation ratio of second phase is represented the total amount of these tissues.In addition, the volume fraction of remaining γ does not have special stipulation at this, for example, can by use with Co as the x-ray source of target, utilize X-ray diffraction α 200}{211}{220} face, γ { the integrated intensity ratio of 200}{220}{311} face is obtained.The anisotropy of material structure is extremely little in the steel of the present invention, and therefore, the volume fraction of remaining γ and area occupation ratio are about equally.With in the second such phase particle with the particle of ferrite crystal boundary adjacency more than three second particle mutually as the triple point place that is present in the ferrite crystal boundary, obtain its area occupation ratio.In addition, under situation about existing, in a single day both contact parts reach the width identical with crystal boundary between second phase, then count respectively, under than the wideer situation of the width of crystal boundary, count as a particle when contacting with certain width.

The area occupation ratio of second phase: 3～15%

In order to obtain low YP when guaranteeing good ageing resistance, the area occupation ratio that need make second phase is more than 3%.If second phase ratio is lower than 3%, though then can access high BH, the ageing resistance variation, YP rises.In addition, the area occupation ratio of second phase surpasses at 15% o'clock, and YP rises and BH reduces.Therefore, the area occupation ratio of second phase is made as 3～15% scope.In order to obtain lower YP when obtaining higher BH, the area occupation ratio of second phase is preferably below 10%, more preferably below 7%.

The area occupation ratio of martensite and remaining γ is with respect to the ratio of second phase area: surpass 70%

After annealing, implement in the thermal history of slow refrigerative CGL,, then generate small perlite or bainite in abutting connection with ground, produce the rising of YP, the variation of ageing resistance, the reduction of BH with martensite if [Mneq] do not optimize.By optimizing the generation that [Mneq] suppresses perlite or bainite, the area occupation ratio that makes martensite and remaining γ surpasses 70% with respect to the ratio of the second phase area rate, thus, even a spot of second phase ratio of specialized range among the present invention also can be guaranteed sufficient ageing resistance.In addition, in order to give low YP and high BH, the area occupation ratio that need make martensite and remaining γ surpasses 70% with respect to the ratio of the second phase area rate.

The ratio of area occupation ratio that is present in second phase at crystal boundary triple point place in the second phase area rate: more than 50%

In order to obtain low YP and high BH, second phase ratio and martensite and remaining γ need be controlled in the above-mentioned scope with respect to second mutually the area occupation ratio, but like this and insufficient, also need to optimize the location of second phase.That is, has the second identical phase ratio, identical martensite and remaining γ steel plate, the YP height of second mutually small and second steel plate that generates unevenly mutually with respect to the ratio of the area occupation ratio of second phase.Relative therewith, find second mainly even mutually, the thick ground dispersive steel plate at crystal boundary triple point place, YP is low and BH is high.Find in addition, in order to obtain so low YP and high BH, as long as the ratio of area occupation ratio that is present in second phase at crystal boundary triple point place in the second phase area rate is controlled at more than 50%.Therefore, the ratio of area occupation ratio that is present in second phase at crystal boundary triple point place in the second phase area rate is made as more than 50%.

Its reason is still indeterminate, but is presumed as follows.Promptly, find when organizing by the bottom of the various steel plates of tem observation, the second mutually small and steel plate that generate unevenly, martensite not only disperses with the point range shape unevenly at the crystal boundary triple point place of ferrite crystal grain but also on the specific crystal boundary beyond the triple point, intersperses among the narrow and small zone, interval between the martensite.Learn that clearly the dislocation that produces when quenching around martensitic is imported in a large number, but martensite during with intensive generations of point range shape, martensite importing on every side dislocation regional overlapped.Think that surrender produces around martensite in the complex tissue steel that is made of ferrite and martensite, if dense distribution between the martensite, then such distortion from the low-stress at initial stage that produces around the martensite is hindered, and YP uprises.Think to be present in equably mutually in the steel plate at triple point place of crystal boundary that martensite has fully big interval between mutually and disperses, easily from such viscous deformation that produces around martensitic second.In addition, though agnogenio, in the second mutually homodisperse steel plate, in prestrain 2% and the distortion after 170 ℃ are implemented 20 minutes thermal treatment down, see tangible yield-point phenomenon, i.e. the phenomenon that produces significantly of upper yield point and lower yield point, BH uprises.

Such tissue morphology can be by adding chilling that P or B, process of cooling after hot rolling implement specialized range, carrying out low temperature and batch and obtain.

3) create conditions

Steel plate of the present invention, as mentioned above, can be by following manufacture method manufacturing: will have the steel billet that the one-tenth that is defined as above is grouped into and carry out hot rolling and cold rolling, then in continuous hot galvanizing line (CGL), be higher than 740 ℃ and be lower than under 840 ℃ the annealing temperature and anneal, from described annealing temperature with the cooling of 2～30 ℃/second average cooling rate after, be impregnated into carry out in the zinc-plated bath zinc-plated, zinc-plated back is cooled to below 100 ℃ with 5～100 ℃/second average cooling rate, or, be cooled to below 100 ℃ at the average cooling rate with 5～100 ℃/second after the Alloying Treatment in the further Alloying Treatment of implementing coating in zinc-plated back.

Hot rolling

Hot rolling steel billet can carry out by the following method: with the rolling method in steel billet heating back; With without heating and the direct rolling method of the steel billet after the continuous casting; The method that steel billet after continuous casting enforcement short heat treatment is rolled etc.Hot rolling is implemented to get final product according to conventional methods, and for example, billet heating temperature is that 1100～1300 ℃, finishing temperature are Ar ₃Transformation temperature～Ar ₃Transformation temperature+150 ℃, coiling temperature are 400～720 ℃ and get final product.

In the steel of the present invention, compound P and the B of being added with, the γ → α after the hot rolling, perlite, bainitic transformation significantly postpone, and therefore, by hot-rolled condition being controlled in the scope shown below, can access higher BH.

Steel (Mneq:2.4, the 8P+150B that will contain C:0.024%, Si:0.01%, Mn:1.55%, P:0.035%, S:0.003%, sol.Al:0.05%, Cr:0.20%, N:0.003%, B:0.0018% ^*: 0.59, steel of the present invention) and contain C:0.024%, Si:0.01%, Mn:1.85%, P:0.01%, S:0.003%, sol.Al:0.05%, Cr: do not add, N:0.003%, B:0.0008% (Mneq:2.1,8P+150B ^*: steel vacuum melting 0.29, comparative steel), the relation of the speed of cooling after BH and the hot rolling is investigated.When steel of the present invention was made sample, the average cooling rate till making after the hot rolling 640 ℃ changed in 2 ℃/second～90 ℃/second scope.The measuring method of other create conditions, BH is with before identical.It is the results are shown in Fig. 6.

As shown in Figure 6, compare with comparative steel, the BH height of steel of the present invention, and the speed of cooling in hot rolling is to demonstrate extra high BH more than 20 ℃/second the time.In addition, speed of cooling is to demonstrate higher BH more than 70 ℃/second the time.Comparative steel needs very large speed of cooling in order to make BH and to increase, but has improved the Mn equivalent and effectively utilized the steel of the present invention of B, even the chilling of appropriateness also can access the effect that BH is increased.This be because, existing steel needs very large speed of cooling for thick perlite is disappeared, but added B and improved the normal steel of the present invention of Mn, thick perlite disappears and becomes small perlite under the speed of cooling more than 20 ℃/second, becomes the tissue that bainite is a main body under the speed of cooling more than 70 ℃/second.As a result, second after the annealing disperses more equably at crystal boundary triple point place, and also homogenizing of ferrite crystal grain, thereby BH improves.The control of such speed of cooling need be carried out in the temperature range below 640 ℃.This is because when stopping chilling under higher temperature, can generate thick perlite when slow cooling thereafter.In addition, preferably coiling temperature is made as 400～620 ℃ scope.This is because when coiling temperature is high, can generate thick perlite during same keeping for a long time after batching.Therefore, in the steel of the present invention, preferably, under 400～620 ℃, batch then being cooled to temperature below 640 ℃ with the average cooling rate more than 20 ℃/second after the hot rolling.

In order to obtain the coating surface quality of the beauty that outside plate uses, preferred billet heating temperature is below 1250 ℃, for remove surface of steel plate generate once, the secondary scale, preferably fully carrying out de-scaling and making finishing temperature is below 900 ℃.In addition, when comprising the steel of the present invention of C, Mn, P according to the ordinary method manufacturing, the r value of rolling right angle orientation uprises, the r value step-down of rolling 45 degree directions.That is, producing Δ r is+0.3～0.4.In addition, the YP (YP of rolling 45 degree directions _D) with the YP (YP of rolling direction _L) or the YP (YP of rolling right angle orientation _C) compare high 5～15MPa.From the viewpoint of the intra-face anisotropy that reduces r value and YP, preferably the average cooling rate after the hot rolling is more than 20 ℃/second, or finishing temperature is below 830 ℃.Thus, Δ r can be suppressed to below 0.2, with YP _D-YP _CBe suppressed to below the 5MPa, thereby can suppress surface strains around the handle of car door effectively.By making the average cooling rate after the hot rolling is more than 70 ℃/second, Δ r can be suppressed to below 0.15, therefore, preferably the speed of cooling after the hot rolling is controlled in this scope.

Cold rolling

In cold rolling, making rolling rate is 50～85% to get final product.Thereby viewpoint deep drawing quality is improved from the r value is improved, preferably making rolling rate is 65～73%, from the viewpoint of the intra-face anisotropy that reduces r value and YP, preferably making rolling rate is 70～85%.

CGL

For the steel plate after cold rolling, in CGL, implement annealing and plating processing or after plating is handled, further implement Alloying Treatment.Annealing temperature is made as and is higher than 740 ℃ and be lower than 840 ℃.In the time of below 740 ℃, the solid solution of carbide is insufficient, can not stablize the area occupation ratio of guaranteeing second phase.In the time of more than 840 ℃, can not obtain enough low YP.That implements in common continuous annealing is higher than in 740 ℃ the temperature range, and soaking time is to get final product more than 20 seconds, more preferably more than 40 seconds.

After the soaking, be cooled to remain on usually the temperature of 450～500 ℃ zinc-plated bath from annealing temperature with 2～30 ℃/second average cooling rates.When speed of cooling was lower than 2 ℃/second, in 500～650 ℃ temperature range, perlite generated in a large number, can not obtain fully low YP.On the other hand, speed of cooling is during greater than 30 ℃/second, and near 500 ℃ in plating bath before and after the dipping, γ → α phase transformation is significantly carried out, and the second phase microminiaturization and the area occupation ratio that is present in second phase at crystal boundary triple point place tail off, and YP rises.

Then, be impregnated into carry out in the zinc-plated bath zinc-plated, as required also can be by further in 470～650 ℃ temperature range, keeping 30 seconds with the interior Alloying Treatment of implementing.The steel plate that existing [Mneq] do not optimize, by implementing such Alloying Treatment, the remarkable variation of material, but steel plate of the present invention, the rising of YP is little, can access good material.

Carry out under the situation of Alloying Treatment after zinc-plated, the average cooling rate with 5～100 ℃/second after the Alloying Treatment is cooled to below 100 ℃.When speed of cooling is lower than 5 ℃/second, near 550 ℃, generate perlite, in addition, in 400 ℃～450 ℃ temperature range, generate bainite, thereby YP is risen.On the other hand, speed of cooling is during greater than 100 ℃/second, and the martensitic self-tempering that produces in the cooling is insufficient continuously, the martensite too hardization that becomes, thus YP rises and ductility reduces.Under the situation with the equipment that can carry out the tempering modifier treatment, from the viewpoint of low YPization, the overaging that also can implement under the temperature below 300 ℃ 30 seconds～10 minutes is handled.

From the viewpoint of the press formability stabilizations such as planarization of the adjustment that makes surfaceness, plate shape, can implement skin pass rolling to resulting steel plate galvanized.At this moment, from the viewpoint of low YP, high E1ization, preferred smooth elongation is 0.2～0.6%.

Embodiment

After the steel melting with the steel numbering system A～A0 shown in table 1 and the table 2, cast thick steel billet continuously into 230mm.

After this steel billet is heated to 1180～1250 ℃, implement hot rolling with the finishing temperature of 820～890 ℃ of scopes.Then, shown in table 3 and table 4, be cooled to below 640 ℃ with 15～80 ℃/second average cooling rates, and ℃ to batch coiling temperature CT:400～650.Resulting hot-rolled sheet is implemented cold rolling with 70～77% rolling rate, obtained the cold-reduced sheet of thickness of slab 0.75mm.

Resulting cold-reduced sheet was being annealed 40 seconds among the CGL, under the annealing temperature AT shown in table 3 and table 4, with the average cooling rate that begins from annealing temperature AT till the plating bath temperature is that a speed of cooling shown in table 3 and the table 4 is cooled off, and is impregnated into pot galvanize and carries out in bathing zinc-plated.When not carrying out Alloying Treatment after zinc-plated, zinc-plated back is cooled to below 100 ℃ in the mode that begins average cooling rate till 100 ℃ from the plating bath temperature and reach the secondary speed of cooling shown in table 3 and the table 4; When carrying out Alloying Treatment after zinc-plated, be cooled to below 100 ℃ to begin the mode that average cooling rate till 100 ℃ reaches the secondary speed of cooling shown in table 3 and the table 4 from the alloying temperature after the Alloying Treatment.Zinc-platedly bathing temperature: 460 ℃, carry out under the condition of Al:0.13% in bathing, Alloying Treatment is following carries out: plating bath dipping back is heated to 480～540 ℃ with 15 ℃/second average rate of heating, and the mode that reaches with Fe content in the coating in 9～12% the scope kept 10～25 seconds.With the coating adhesion amount is one-sided 45g/m ²Mode the two sides is adhered to.It is 0.2% temper rolling that resulting hot-dip galvanizing sheet steel is implemented elongation, and cuts sample.

For resulting sample, with method as discussed previously, investigated second phase area occupation ratio, martensite and remaining γ area occupation ratio with respect to the ratio of the second phase area rate (second in mutually martensite and the ratio of remaining γ), second ratio (second in mutually the ratio of second phase that is present in crystal boundary triple point place) of area occupation ratio that is present in second phase at crystal boundary triple point place in mutually.In addition, observe by SEM structure of steel is separated by kind, measure the volume fraction of remaining γ with the previous described method of utilizing X-ray diffraction.In addition, from cutting the JIS5 test film, implement tension test (according to JIS Z2241) with the rectangular direction of rolling direction, and to YP, TS, YR (=YP/TS), E1 estimates.

Giving elongation to test film same as described above is 2% prestrain, then 170 ℃ of thermal treatments of implementing 20 minutes down.With give after 2% the prestrain stress with in the difference of 170 ℃ of YP after implementing thermal treatment in 20 minutes down as BH.In addition, the mechanical characteristics after keeping 3 months under 50 ℃ is similarly investigated, and estimated ageing resistance with the generation of YPE1.

In addition, use simulation edge adds the Ministry of worker, spot welding part tectosome is on every side estimated the erosion resistance of each steel plate.Promptly, with two overlapping spot weldings of going forward side by side of resulting steel plate, make the state that reaches driving fit between the steel plate, further implement chemical conversion processing, the electrodeposition coating of the coating process in the simulation real vehicle, under SAE J2334 corrosive cycle condition, carry out corrosion test then.The electrodeposition coating thickness is made as 20 μ m.For through the corrosion sample after 90 circulations, remove corrosion product, obtain the reduction of the former thickness of slab that thickness ratio measures in advance, with it as the corrosion decrement.

Show the result in table 3 and table 4.

The steel plate of example of the present invention adds steel with existing C r and compares, and the corrosion decrement significantly reduces, and with heavy addition the steel of Mn or the steel that the steel that added Mo is in a ratio of identical TS level, have low YP and high BH.That is, existing heavy addition steel AF, the AG of Cr, the corrosion decrement big, be 0.45～0.75mm.Relative therewith, the corrosion decrement of steel of the present invention is 0.25～0.37mm, significantly reduces.In addition, though not record in the table, but for existing 340BH (0.002%C-0.01%Si-0.4%Mn-0.05%P-0.008%S-0.04%Cr-0.06%sol.A l-0.0018%N-0.0008%B steel), also carried out the evaluation of erosion resistance together, it is 0.32～0.37mm that the result is corroded decrement.Therefore as can be known, steel of the present invention has and has the roughly equal erosion resistance of steel now.Wherein, the low and heavy addition of Cr amount the steel E of P and steel I and then remove the also compound steel R that adds Cu, Ni reduction Cr, the heavy addition P, added the steel V of Ca etc., erosion resistance is good especially.

Like this when reducing Cr to improve erosion resistance control Mn equivalent and suppress Mn heavy addition and with 8P+150B ^*Be controlled at the steel in the specialized range, can suppress the generation of perlite, bainite, and be present in the ratio height of second phase at crystal boundary triple point place, when keeping low YP, can access high BH.For example, steel A, B, C, D, E have all obtained the high BH more than the 55MPa in the low YP below keeping 220MPa.Particularly steel A, B, C, D, E according to this order, make 8P+150B in the time of the addition of inhibition Mn ^*Increase, second the ratio of second phase that is present in crystal boundary triple point place in mutually increases, and BH significantly increases when keeping low YP.In addition, by steel F, H as can be known, P has added more than 0.015%, B has added the steel more than 0.0003% and can access such characteristic.By steel C, I, J as can be known,, can access low YP,, can access lower YP,, can access further low YP by making [Mneq] 〉=2.4 by making [Mneq] 〉=2.3 by making [Mneq] 〉=2.2.

In addition, these steel, by make speed of cooling after the hot rolling be more than 20 ℃/second, more preferably more than 70 ℃/second, second the ratio of second phase that is present in crystal boundary triple point place in mutually increases, BH further increases.In addition, the component steel of the scope of the invention is as long as annealing temperature, speed of cooling, secondary speed of cooling in specialized range, then can access the tissue morphology of regulation, thereby obtain good material.

In addition, steel K, L, M, N that C amount is increased successively and do not control Mn, 8P+150B ^*Existing steel compare, when same strength level, also have low YP and high BH.

In addition, being controlled at second phase ratio in the specialized range and having reduced the steel of the present invention of the ratio of perlite, bainite, is below 0.3% at the generation of 50 ℃ of YPEl after keeping 3 months down, and ageing resistance is all good.

In addition, controlled area occupation ratio, martensite and the remaining γ of second phase steel of the present invention, also had high E1 concurrently with respect to the dispersing morphology of the ratio of the total area rate of second phase, second phase.

Relative therewith, 8P+150B ^*Steel X, the Y of You Huaing not, YP height and BH are low.The excessive steel AC that has added P, though the BH height, YP is also high.Heavy addition the steel AH of Mo, the YP height.Steel AI, AJ, AK, AL that Ti, C, N, [Mneq] do not optimize, YP is all high.In addition, the ageing resistance of steel AJ, AK, AL is also insufficient.

Utilizability on the industry

According to the present invention, can make excellent corrosion resistance, YP is low and BH is high and ageing resistance is also good high-strength hot-dip zinc-coated steel sheet with low cost.High-strength hot-dip zinc-coated steel sheet of the present invention has superior corrosion resistance, good anti-surface strains, good shock-resistance, good ageing resistance concurrently, therefore, can realize high strength, the thin-walled property of trolley part.

Claims

1. a high-strength hot-dip zinc-coated steel sheet is characterized in that,

One-tenth as steel is grouped into, in quality %, contain C: surpass 0.015% and be lower than 0.100%, following, the Mn of Si:0.3%: be lower than 1.90%, P:0.015% is above and 0.05% following, S:0.03% is following, sol.Al:0.01% is above and 0.5% following, N:0.005% is following, Cr: be lower than 0.30%, B:0.0003% is above and 0.005% following and Ti: be lower than 0.014%, and satisfy 2.2≤[Mneq]≤3.1 and 0.42≤8[%P]+150B ^*≤ 0.73, surplus is made of iron and unavoidable impurities;

Tissue as steel, have ferrite with second mutually, the area occupation ratio of second phase is 3～15%, the area occupation ratio of martensite and remaining γ surpasses 70% with respect to the ratio of the second phase area rate, the ratio of area occupation ratio that is present in second phase at crystal boundary triple point place in the second phase area rate is more than 50%

2. a high-strength hot-dip zinc-coated steel sheet is characterized in that,

One-tenth as steel is grouped into, in quality %, contain C: surpass 0.015% and be lower than 0.100%, following, the Mn of Si:0.3%: be lower than 1.90%, P:0.015% is above and 0.05% following, S:0.03% is following, sol.Al:0.01% is above and 0.5% following, N:0.005% is following, Cr: be lower than 0.30%, B:0.0003% is above and 0.005% following, Mo:0.1% is following and Ti: be lower than 0.014%, and satisfy 2.2≤[Mneq]≤3.1 and 0.42≤8[%P]+150B ^*≤ 0.73, surplus is made of iron and unavoidable impurities;

3. high-strength hot-dip zinc-coated steel sheet as claimed in claim 1 or 2 is characterized in that, satisfies 0.48≤8[%P]+150B ^*≤ 0.73.

4. as each described high-strength hot-dip zinc-coated steel sheet in the claim 1 to 3, it is characterized in that at least a in also containing below the V:0.4%, below the Nb:0.015%, below the W:0.15%, below the Zr:0.1%, below the Cu:0.5%, below the Ni:0.5%, below the Sn:0.2%, below the Sb:0.2%, below the Ca:0.01%, below the Ce:0.01%, below the La:0.01% in quality %.

5. the manufacture method of a high-strength hot-dip zinc-coated steel sheet, it is characterized in that, to have the steel billet that each described one-tenth is grouped in the claim 1 to 4 and carry out hot rolling and cold rolling, be among the CGL at continuous hot galvanizing line then, be higher than 740 ℃ and be lower than under 840 ℃ the annealing temperature and anneal, cooling off from described annealing temperature average cooling rate with 2～30 ℃/second before be impregnated into the zinc-plated bath, be impregnated into then carry out in the zinc-plated bath zinc-plated, zinc-plated back is cooled to below 100 ℃ with 5～100 ℃/second average cooling rate, perhaps, zinc-plated back is further implemented the Alloying Treatment of coating and is cooled to below 100 ℃ at the average cooling rate with 5～100 ℃/second after the Alloying Treatment.

6. the manufacture method of high-strength hot-dip zinc-coated steel sheet as claimed in claim 5 is characterized in that, when carrying out hot rolling, being cooled to below 640 ℃ with the average cooling rate more than 20 ℃/second after the hot rolling, batches under 400～620 ℃ then.